This invention relates to improved cathode materials for rechargeable lithium ion batteries. The cathode material contains Ni, Mn and Co, is surface treated and has a modified composition which shows improved cycle stability during long term cycling in full cells, especially at elevated temperatures. The modified material also shows an improved stability when cycled at high voltages. Additionally the content of soluble base is low.
A cathode of a lithium rechargeable battery is usually a powderous material capable to reversibly intercalate and de-intercalate lithium. The powder is deposited as an electrode film onto a metal foil acting as a current collector. The electrode film often contains more than 95% of the cathode material as well as small amounts of binder and conductive additive. The thus prepared positive electrode is stacked or wound with a separator and a negative electrode film (anode). Typically the negative electrode film contains carbon. Finally the stacked or wound cathode-separator-anode-roll is inserted into a case, electrolyte is added and the case is sealed, resulting in a full cell.
Historically LiCoO2 was the dominating cathode material for rechargeable lithium batteries. Recently the so-called NMC cathode materials replace LiCoO2 in many applications. “NMC” is an abbreviation of nickel-manganese-cobalt and is used for lithium transition metal based oxides, where the transition metal is a mixture of basically Ni, Mn and Co, and having roughly the stoichiometry LiMO2, where M=NixMnyCoz. Additional doping is possible, typical doping elements are Al, Mg, Zr etc. The crystal structure is an ordered rocksalt structure, where the cations order into 2 dimensional Li and M layers. The space group is R-3M. There are many different compositions possible, often categorized and named after their nickel manganese and cobalt content. Typical NMC based materials are “111” where M=Ni1/3Mn1/3Co1/3, “442” with M=Ni0.4Mn0.4Co0.2, “532” with M=Ni0.5Mn0.3Co0.2, “622” with M=Ni0.6Mn0.2Co0.2 etc.
The composition of NMC is normally characterized by (1) the Li:M molar ratio, (2) the cobalt content Co/M, and (3) the so-called nickel excess (Ni—Mn)/M, as follows:
(1): Generally the Li to M stoichiometric ratio is near to, but often not exactly unity. If Li:M increases Li replaces M on M layer sites and the structure can—simplified—be written as Li1[M1−xLix]O2 or Li1+xM1−xO2, where Li:M=(1+x)/(1−x). Typical Li:M is close to 1.10 for “111” and “442”; and close to 1.02 for “622”. One effect of increasing the Li:M stichiometric ratio is that the cation mixing is changed. Cation mixing refers to the fact that the actual crystal structure is not exactly layered LiMO2 or Li1[M1−xLix]O2, but rather {Li1−x′Mx′}[M1−yLiy]O2, where x′ stands for M atoms that are located on Li layer sites, which results in so-called “cation mixing”. In NMC we typically observe some degree of cation mixing. It is believed that cation mixing hinders a very fast Li diffusion, thus limiting the rate performance and power of NMC cathode materials.
(2): The cobalt content is important for stabilizing a well layered structure. If the cobalt content Co/M is less than about 10-20 mol %, then often increased cation mixing is observed. On the other hand cobalt is more expensive than Ni and Mn, and hence many NMC cathode materials have a cobalt content of 15-33 mol % as an optimized value.
(3): The nickel excess is an important stoichiometric parameter. As (Ni—Mn)/M increases the reversible capacity increases, but at the same time the cathode material becomes increasingly difficult to prepare. “111” with Ni excess=0 is a robust material that can be easily prepared by sintering e.g. a mixed NMC hydroxide (for example precipitated from an aqueous solution of Ni—, Mn—, and Co sulfates) together with cheap Li2CO3 as lithium source in a flow of air at high throughput. High Ni cathodes—like “811”—often require sintering in pure oxygen, at lower throughput, and instead of Li2CO3 the more expensive and more difficult to handle LiOH*H2O must be used as Li source. With increasing Ni excess it is increasingly difficult to control the content of soluble base. High Ni material tend to have a very high base content, which is a severe issue for many applications.
Background of soluble base content: when a cathode material LiMO2 is inserted in water then the pH of the water increases. The pH increase is caused by
(a) dissolution of surface impurities like Li2CO3 and LiOH*H2O,
(b) reactions of surface molecules/surface groups with water, and
(c) ion exchange reactions where lithium present in the top layers of the bulk of the material is exchanged for protons following the reaction Li++H2O→H++LiOH. The pH increase is utilized in techniques called “pH titration” for determining the soluble base content. In a typical pH titration experiment a certain amount of cathode powder (e.g. 2.5 g) is added to an amount of water (e.g. 100 ml), the suspension is stirred during a fixed time (e.g. 10 min), followed by filtering. The pH titration itself shows a pH profile as function of an amount of added acid
(e.g. 0.1 M HCl). In many cases the pH profile can be easily explained based on the presence of the mixture of carbonate (Li2CO3) and hydroxide (LiOH). By certain manipulations the contents of LiOH base, Li2CO3 base, BT (total base) etc. are obtained, as is explained in e.g. WO2012/107313, which is incorporated here by reference.
pH titration is a sensitive tool to investigate the surface properties of cathode materials. Since the cell performance of cathode materials depends on its surface properties—because parasitary reactions occur at the electrolyte/particle interface—it could be expected—and experience has shown—that pH titration is a powerful tool to design cathode materials. As an example, in WO2012/107313 it is shown that results of pH titration correlate well with many full cell properties and cell making properties. A high base content for example correlates with                increased bulging (internal gas evolution) of full cells, where typical tests include the measurement of the thickness increase of fully charged cells, and        electrode slurry stability issues with different binders: both in water-based electrode coatings, and in NMP (N-Methyl-2-pyrrolidone) based electrode coatings a high base content causes issues like aluminum foil corrosion (for water based coatings) or slurry viscosity changes (for NMP based coatings).        
Regarding portable applications, NMC is not yet penetrating into the high end portable devices, for example in polymer cells for smartphones. A major reason is that either the volumetric energy density is too low (if the Ni excess is much below 20%) or—if the capacity is sufficient (such as for a Ni excess (far) above 20%)—then the high content of soluble base causes bulging, which cannot be tolerated. In low end portable applications NMC is replacing LiCoO2.
A typical example is cylindrical or prismatic cells for notebooks. The rigid case tolerates some internal gas evolution, so NMC with ≥20% Ni excess can be applied. Typical electrode test schedules include continuous full charge—discharge cycling where the typical target is to exceed 80% of initial reversible capacity after 500 cycles. In some applications which use advanced high voltage LiCoO2 the charge voltage has been increased to 4.35V or even 4.4V. NMC can only challenge LiCoO2 if the NMC can be cycled in a stable manner at 4.35V or even 4.4V in full cells. Summarizing: important properties for NMC cathode materials for portable applications include a high capacity and low bulging with an acceptable cycling stability.
Regarding automotive applications, the batteries are larger than portable batteries and thus more expensive. Therefore, quite generally, the requirements for cathode price, cycle stability and calendar life are much more demanding compared to portable applications. Also, batteries might operate at higher temperatures, so requirements for high T cycling stability are stricter. LiCoO2, due to its cost and limited calendar life, is much less considered as cathode material for automotive applications than NMC. Typical test schedules include continuous full charge discharge—cycling where the typical target is to exceed 80% of initial reversible capacity after 2000 cycles both when cycling at 25° C. as well as when cycling at 45° C. with a charge voltage of 4.2V. Because of tough battery life requirements, a typical charge voltage during application is even lower than 4.2V. NMC which cycles in a stable way at higher voltage might be of interest to increase the charge voltage during its use. An automotive battery contains many cells, controlled by a battery management system. To lower system cost a more simple battery management system is desired. One contribution to the cost is the heating/cooling system which ensures that the cells operates at the appropriate temperature. At low temperatures the battery has insufficient power, whereas at high temperature the cycle stability becomes a concern. Obviously the system cost can be reduced if the automotive cathode materials support stable cycling not only at 25° C. but also at higher temperature.
Some automotive applications (for example hybrid electric vehicles or HEV) require very high power. This is due to the high charge/discharge rates during regenerative braking and acceleration. A high power output requires (1) electronically high conductive electrodes, (2) thin electrodes and (3) cathode materials which support high charge/discharge rates. NMC with a high Ni excess is especially suited for this application, because (1) the electronic conductivity increases with Ni excess, (2) as capacity increases with Ni excess electrodes can be made thinner and (3) often power (for fixed Co stoichiometry) increases with Ni excess. Summarizing: Important properties for NMC cathode materials for automotive applications include high capacity and good cycle stability both at normal (25° C.) as well as at elevated temperature.
An object of the present invention is to provide NMC cathode materials with Ni excess that are showing improved properties required for high end portable and automotive applications.